FTIR-based touch-sensing systems (“FTIR systems”) are optical touch systems that use projection measurements for light that propagates on a plurality of propagation paths inside a light transmissive panel. The projection measurements thus quantify a property, e.g. power, of the light on the individual propagation paths, when the light has passed a touch-sensitive region on the panel. The light propagates by total internal reflection (TIR) inside the panel such that an object that touches the touch-sensitive region causes the propagating light on one or more propagation paths to be attenuated or “frustrated” (FTIR, Frustrated Total Internal Reflection). The projection measurements may be processed to identify the location of touches on the touch-sensitive region, e.g. by triangulation or by advanced image reconstruction techniques.
To achieve a dense grid of propagation paths, the light may be injected into the panel as diverging beams of light, which thus expand in the plane of the panel as they propagate inside the panel from a respective entry port on the panel. By measuring the property of each beam of light at a plurality of spaced apart exit ports on the panel, each beam forms a plurality of propagation paths between the entry port and a number of the exit ports. Such an FTIR system is disclosed in U.S. Pat. No. 7,432,893, which uses revolved prisms to couple diverging beams of light into the panel, and photodetectors directly attached to the panel to detect the propagating light.
In touch-sensitive devices, there is a general trend to avoid attaching components to the front surface. These components may form a frame around the touch-sensitive region and thereby reduce the ratio of the active area (the surface area that is available for touch interaction) to the total area of the touch-sensitive device. Furthermore, if the components protrude from the front surface of the panel, it may be necessary to provide a bezel at the perimeter of the panel to protect and hide the components and possibly any wiring connected to the components. Given the nature of user interaction with touch-sensitive devices, such a bezel may disrupt the user experience and even prevent certain types of interaction. The bezel may also cause dirt and other contaminants to accumulate in the area where the bezel joins the panel. To overcome this problem, it is desirable to design touch systems for flush mount of the panel in the supporting frame of the touch-sensitive device, i.e. such that the front surface of the panel is level with the surrounding frame material. This is also known as “edge-to-edge”.
It is thus desirable to achieve a touch-sensing system that is compact and has a flat front surface, without bezel or other projecting structures, while reducing the impact of ambient light.
Aforesaid U.S. Pat. No. 7,432,893 proposes to suppress the impact of ambient light by attaching the photodetectors to the front surface, such that the photodetectors face away from the ambient light that enters the panel through the front surface. Clearly, such an FTIR system will require a significant bezel to hide and protect the photodetectors and the associated wiring. U.S. Pat. No. 7,432,893 also proposes using large revolved prisms, e.g. attached to the rear surface of the panel, for coupling light into the panel. This adds considerable weight and thickness to the touch system.
The prior art also comprises WO2012/105893 which proposes using a sheet-like microstructured element, e.g. a tape of light transmissive material, on the front or rear surface of panel for coupling light into the panel of an FTIR system. WO2012/105893 also notes that a similar sheet-like microstructured element may be used for coupling light out of the panel. The use of sheet-like microstructured elements may thus serve to reduce the weight and thickness of an FTIR system. However, WO2012/105893 provides no solution to the conflict between providing a flat front surface and suppressing the impact of ambient light that enters the panel through the front surface.